Prior to this application, the applicant submitted a proposal for a position detector to determine the coordinate values of the position specified by the corresponding position indicator through transmission/reception of electromagnetic waves between a sensing unit and the position indicator under U.S. Pat. No. 5,028,745 (hereinafter referred to as the prior application).
The contents of the prior application will be briefly explained below. First, an electromagnetic wave is generated by applying an AC signal to a loop coil in the sensing unit consisting of multiple loop coils juxtaposed in the direction of position detection. The tuning circuit incorporated in the position indicator is driven by this electromagnetic wave. At this time, an electromagnetic wave emitted from the tuning circuit is received by the said loop coil to generate an induced voltage. This is repeated on multiple loop coils in sequence. The amplitude and phase angle of the induced voltage generated at each loop coil or the received signal are detected. With these values, the coordinate value of the specified position is determined.
However, in the above device, at least, all loop coils must be switched to determine the initial position of the position indicator. This requires considerable detection time. In particular, a large sensing unit is required. That is, the device has the disadvantage of requiring more time as the number of loop coils increases.
To detect the amplitude and phase angle of the signal corresponding to the AC signal actually applied to a loop coil from among received signals, the above device uses a ceramic filter which allows only the frequency component corresponding to the frequency of the AC signal to pass. Therefore, if two or more AC signals of different frequencies are used, ceramic filters corresponding to each frequency must be provided. Thus, the device has the disadvantage that it is difficult to simultaneously use multiple position indicators to increase the number of AC signal frequencies.
Additionally, the transient response characteristics of the above ceramic filter are so poor that transmission/reception of electromagnetic waves must be repeated several times for each loop coil to receive a stable output signal. Thus, the device has the disadvantage that the loop coil switching speed cannot be increased because it takes considerable time to select a loop coil.
Furthermore, the said ceramic filter includes a mechanical vibrator, is of a specific size, and requires many analog switches for individual loop coils for loop coil switching. Thus, the device has the disadvantage that component installation area and height cannot be reduced and it is difficult to design the device in IC form.